In general, a discharge reaction of the lead-acid battery produces lead sulfate (PbSO4) through electrochemical reduction and oxidation of lead dioxide (PbO2) and lead (Pb) in sulfuric acid (H2SO4) electrolyte solution; where PbO2 and Pb are active materials of positive and negative electrodes. The produced PbSO4 on the positive and negative electrodes is electrochemically reduced as well as oxidized by charging, thereby not only producing PbO2 and Pb, but also revitalizing H2SO4. The overall reaction of charge/discharge of the lead-acid battery is described as the following formula (1):

The following characteristics are desirable for the separator of the lead-acid battery:
1. Acid-resistant, anti-oxidation and anti-reduction:
Since strong-acid H2SO4 may be used as electrolyte, and also, electrochemical oxidation as well as reduction is repeated on both the positive and negative electrodes through charging and discharging processes, a chemically stable material from which a harmful substance is not soluble should be selected.
2. High ion-conductivity and no possibility of internal short-circuit due to contact between the positive and negative electrodes:
Electrolyte is easy to permeate and diffuse through the separator disposed between the positive and negative electrodes because hydrogen ion H+ and sulfate ion SO4−2 are of electrolytic dissociation in the electrolyte together with active materials PbO2 and Pb of the positive and negative electrodes and are reaction species, whereby the separator is vulnerable to be permeated as well as diffused with the electrolyte. In order to avoid contact between the positive and negative electrodes, it is desirable for the separator to be long and bent in shape, and to have a micro-porous construction. Particles in the active materials of the positive electrode, by charge/discharge cycling, tend to drop from an electrode grid which is an electric collector and active material holder. Thus caution is desirable to avoid this particle drop. To be more specific, a micro-porous sheet made of rubber or resin, and pulp or glass-fiber reinforced with resin, is used as the separator.
A paste type electrode plate process which is in high productivity is widely used, among others, in the lead-acid batteries. When using this type, a synthetic resin sheet having the micro-pore structure and a glass mat are used for the separator. The glass mat is in contact with both sides of the positive electrode, thereby preventing the active material of the positive electrode from dropping. As a result, the number of internal short-circuits is reduced. This example is substantially effective when the electrode plate is used in a vented type car battery which is exposed to violent external forces such as vibration, shock, and acceleration.
Recently, Japanese utility model H07-34555 discloses another idea as follows which further reduces the internal short-circuit. An envelope type separator made from micro-porous synthetic resin sheet includes a glass mat which is broader than the separator, being laid on the separator, wherein the separator is folded so that side portions of the synthetic resin sheet may contact each other. Both edges where the synthetic resin sheet side portions are contacting and the outer edges of the glass mat adhere in order to form an envelope.
This separator is also developed to be used in a vented type battery, where the negative electrode is accommodated into the envelope, and the positive electrode is disposed to contact an outer surface of the glass mat. The electrode group thus formed is incorporated in a cell container, thereby forming the cell.
In order to make maintenance of the lead-acid battery easy, a sealed lead-acid battery has been widely used. Oxygen (O2) gas generated from the positive electrode by over-charging is eliminated by an oxygen cycle reaction on the negative electrode. The same process is seen also in a sealed nickel-cadmium battery.
The separator of the sealed lead-acid battery desirably has the following functions other than the above:
(a) O2 gas, generated from the positive electrode during a period between an end of charging and over-charging of the positive electrode, flows to the negative electrode with ease because of excellent ventilation, and the electrolyte is prevented from being fluid.
(b) The electrolyte pertinent to charge/discharge is retained as much as possible around the positive and negative electrodes.
In order to turn the electrolyte to a non-fluid condition, two methods are available:
(1) Retainer method: The electrolyte is absorbed into the positive and negative electrodes as well as the separator so that the electrolyte turns into the pore of solid substance.
(2) Gelled method: The electrolyte turns into gell with colloidal silica.
The retainer method is now mostly used, and it is sometimes called “Absorbed method” or “Starve method.” The retainer method adjusts a quantity of the electrolyte so that ventilation works in some part when the electrolyte is absorbed into the mat type separator and turned into solid. A mat sheet made from fine glass fiber is widely used as a separator satisfying the above functions. This separator made from the mat type glass fiber is a kind of non-woven cloth that can include short fibers made of borosilicate glass having a diameter of not more than 1 μm.
FIG. 3 shows a typical structure of an electrode group used in a conventional sealed lead-acid battery. Mat type glass fiber separator 3 is folded to form a U-shape, and negative electrode 1 is inserted therein, then the outer side of the folded separator 3 is nipped by positive electrode 2, and whereby the electrode group is formed.
When using electrodes of the same dimension, a number of plates is increased in step with capacity, and the electrode group is formed in the same manner. In general, the number of negative electrodes 1 is larger than that of positive electrodes 2 by one. The electrode group is accommodated into the cell container, and the most appropriate quantity of electrolyte is poured therein. Finally, the cell container and its cover are sealed to complete the cell.
In this case, the separator 3 is shaped longer than both sides and upper ends of the negative electrode 1 and positive electrode 2 in order to avoid an internal short-circuit. This structure finds no problem when used in a battery of small size, relatively low capacity, and the positive and negative electrodes with a casting grid having an outer frame; however, when used in a car battery or an electric vehicle (EV) battery, they are relatively high capacity, and in a sealed lead-acid battery, internal short-circuits sometimes occur, because these batteries are exposed to violent outer forces such as vibration, shock, and acceleration. When an expanded grid which has no outer frame on both edges is used in the electrodes, an internal short-circuit often occurs particularly in cycling of charge and deep discharge, because the active materials expand in step with increasing cycles of charge/discharge, which expands an edge of the positive electrode to overgrow the edge of the separator, and whereby the positive electrode makes contact with the negative electrode.
The envelope type separator of which both sides are sealed has been thus proposed in order to solve the above problem; however, it has been difficult to tie both edges of the separator comprising a mat type sheet made of glass fiber while keeping high productivity. Thus felt type non-woven cloth is adopted, which is made from acid proof and thermoplastic synthetic fiber such as polyester or polypropylene, and fine powder of acid clay as well as fine glass fiber mixed therein. Namely, the felt-type non-woven cloth of which dimension is large enough to fold the electrodes therein is doubled-back to form a U-shape, and both edges thereof are heat-sealed to make the envelope type separator. Then the electrode is inserted into this envelope. An example of this method is disclosed in Japanese Patent Laid-open H07-60676, where an envelope separator is used, which comprises felt-type non-woven cloth made from thermoplastic synthetic fiber such as polypropylene and fine glass fiber evenly dispersed therein. Although this separator has two advantages including (a) retaining electrolyte and (b) good heat-sealing, it has less ability of retaining electrolyte than the mat-type separator purely made from glass fiber, and yet, ventilation is not enough since the diameter of synthetic fiber is in general larger than that of glass fiber. While increasing the glass fiber content in order to improve the retainability of electrolyte, tightness of heat-sealing lowers, i.e. these two factors are in a trade-off relation. Therefore, the separator disclosed in H07-60676 does not work well in a sealed lead-acid battery with the glass fiber content of 10–25 wt % defined in H07-60676.